1. Field of the Invention
The present invention relates generally to a system used to control the humidity and temperature of a sample being tested in a thermogravimetrical analyzer (TGA). More particularly, the present invention relates to a humidity-controlled chamber designed to control the atmosphere of a TGA's sample and reference chambers simultaneously to an equal specific relative humidity level and to an equal specific temperature.
2. Background Information
A TGA is generally used to measure the decomposition of a sample as a function of temperature. The device heats a sample and measures the weight gain or loss during the process. In a typical device, a sample is suspended from a very sensitive balance. When heated, the sample undergoes a physical change. The weight versus temperature and weight versus time is then recorded and plotted for each sample. TGAs are described, for example, in U.S. Pat. No. 5,165,792, which is incorporated by reference herein. TGAs are unlike other thermal analysis apparatus (such as dynamic mechanical analyzers and some differential scanning calorimeters, for example), because the sample in the TGA needs to interact with the atmosphere.
Moisture is constantly present in our environment and can affect solid state materials due to its influence on their structural phase. This may be due to the formation of a hydrate phase or an induced amorphous to crystalline phase transition. These moisture induced changes can affect mechanical properties and can also cause chemical interactions. The effect of moisture on pharmaceutical materials, for example, can alter yield, purity, and dosage calculations.
Consequently, it is desirable to analyze materials in environments where the moisture level or humidity is controlled in addition to the temperature. An apparatus for controlling the environment of a sample for microscopic, spectroscopic, or crystallographic analysis is described in PCT Application Serial No. WO 01/92460 (the “'460 application”) by Levoguer et al., entitled “Controlled Sample Environment for Analytical Devices.” This apparatus comprises a sample chamber that is fed by a gas stream having a known vapor content, which is generated by mixing two gas streams, one substantially saturated in a volatile substance and one substantially free of the volatile substance. The temperature of the apparatus, and particularly of the sample chamber, is accurately controlled and regulated by a temperature controller.
A device for thermogravimetric measurement that includes humidity control is detailed in U.S. Pat. No. 5,669,554 (the “'554 patent”) by Nakamura et al. entitled “Humidity Control Thermal Analyzer” and in U.S. Patent Application Ser. No. US 2002/0080848 A1 (the “'848 application”) by Nakamura et al. entitled “Automatic Humidity Step Control Thermal Analysis Apparatus.” The device comprises a chamber for generating water vapor, a sample chamber, a heat insulating pipe, and a detector. The '554 patent and the '848 application differ primarily in their methods of controlling the sample chamber temperature and their signal control and detection.
The apparatus described in the '460 application has not been adapted for use with TGAs. The device detailed in the '554 patent and the '848 application has not been designed to work easily for standard thermal analysis. In view of the foregoing, it can be appreciated that a substantial need exists for apparatus, systems, and methods that can easily be used to adapt a standard TGA for humidity-controlled experiments in addition to standard thermal analysis.